1. Field of the Invention
This invention relates to a method and apparatus for imparting a simple contour to a workpiece and, more particularly, to a method and apparatus for shot peening a wing skin to form a simple contour which matches the chordwire curvature of an aircraft wing.
2. Description of the Prior Art
The application of simple curvatures to aircraft skins by shot peen forming is not new. It has been done with air nozzle peening for many years as exemplified by U.S. Pat. No. 2,701,408; and by centrifugal shot throwing wheels as exemplified by U.S. Pat. No. 3,705,510. However, the use of shot peening in a metal part as taught by these and other state of the art patents encourages the formation of simple and compound contours. In the typical peening process, a flat piece of metal is peened to form a predetermined shaped contour which approximates the chordwise curvature of a desired aircraft wing. Because the shot consists of spherical elements, their striking of the treated metal part causes the application of compressive stresses to the metal part along axes in all directions, thereby resulting in the curving or contouring of the part in all directions. This peen shaping is known as hemispheric shaping and is desirable for some applications. Accordingly, a compound contour is imparted to the metal skin, namely the skin develops a spanwise curvature in addition to the more desirable chordwise curvature. U.S. Pat. No. 4,329,862 is exemplary of a complicated method for utilizing and propagating compound contours throughout a flat metal sheet to more closely approximate the exact shape of an aircraft wing. However, devising a system which controls and encourages both chordwise and spanwise curvature is extremely complicated and requires extensive trial and error testing to develop appropriate banks of information which can be later programmed into a computer to control the shot peening equipment. It is, therefore, highly desirable to have a method which easily treats the workpiece and develops only chordwise curvature. In response thereto and in applications where it is necessary to inhibit or prevent compound contour formation, pre-stressing or clamping the metal part in an over-curved condition during peening can be an effective preventive method. Further, some metal parts are provided with longitudinal stiffeners which discourage compound curvature by supplying increased strength to the metal part for withstanding the multi-directional compressive forces of the peening process. Additionally, after compound contours have been formed in the peened metal part, subsequent touch-up peening can also be utilized. Touch up peening is substantially a trial-and-error method employed to correct localized compound contouring by peening various locations on the part in an attempt to reverse some of the undesirable growth and compound curvature which had been previously imparted thereto as described above. Unfortunately, such methods of correction are cumbersome and time consuming and extremely expensive, especially when the costs of the additional touch up peening are included.